Oils refer to crude oils which have high specific gravity and viscosity and are therefore difficult to extract commercially because they do not readily flow. Typically, these oils will have viscosities greater than 1000 mPa-s (centiPoise) or specific gravities greater than 0.934 kg/m3 at 15.5° C. (60° F.) (i.e. less than 20 API). Oil wet reservoirs are oil deposits where the oil is the rock wetting fluid. Typically, these reservoirs have poorer recoveries due to the oil's strong adherence to the reservoir rock. There has long been sought a means to accelerate oil production processes by permitting the oil to flow more readily thereby increasing the rate of return on capital and decreasing the financial risk of such oil production projects.
One approach to oil extraction involves the use of steam in a thermal stimulation to facilitate oil extraction. Steam raises the temperature of the oil and thereby reduces its viscosity and allows it to flow more easily. The two main traditional approaches used in steam recovery systems have been “huff and puff’ (i.e., cyclic steaming) and steam floods. Steam stimulation is subject to a number of problems, including heat losses during injection, clay swelling problems, thief zones, emulsions, capillary surface tension effects and lack of confinement for shallower zones. Further, injecting steam creates water (condensate) in the formation which is much less viscous than oil and which will therefore be preferentially produced due to relative permeability effects. Preferential production of water makes the oil production or recovery more difficult.
Another approach to oil extraction is steam assisted gravity drainage (SAGD). SAGD begins with the formation of a steam chamber in the formation. The steam is injected at the chamber surface. The heated oil flows down the walls of the chamber under the influence of gravity and drains into the production well, thereby increasing the size of the chamber. SAGD employs the countercurrent flow of steam upwards into the reservoir and oil down and out of the reservoir, which in certain situations can be relatively efficient, and provide oil production rates high enough to provide favorable economics. There are many possible SAGD geometries including single well (injection and production from the same well) and dual or multiple well. The wells may be either horizontal or vertical. Generally horizontal wells are favored by producers because they offer a greater (longer) exposure to the pay zone and thereby offer increased production rates for highly viscous oils.
Yet another approach is the vapor extraction (VAPEX) process, which proposes to combine a heated solvent (propane) with hot water heated at surface to provide downhole heat. Because of the use of hot water, this process suffers from the problems mentioned above (countercurrent heat exchange, formation damage problems with clays, emulsions, capillary pressure, water treatment, water supply, reduced oil relative permeability due to high water saturations and the like).
Oil wet reservoirs often require surfactants to alter the wettability of the reservoir. When added to a flooding mechanism, the oil can be removed in a more typical fashion. Unfortunately, incorrect application of surfactants can damage a reservoir beyond repair.
Existing heated solvent processes and Steam-assisted Gravity Drainage (SAGD) processes have typically been avoided for extraction from reservoirs that contain mobile water, as the mobile water has been thought to be detrimental to all forms of enhanced oil recovery (EOR). Thin pay zones have not been considered for SAGD due to associated high heat losses to the surrounding rocks.
In addition, existing heated solvent processes do not work in the carbonate reservoirs due to substantially higher reservoir pressure required which precludes the use of condensing vapor or condensing solvent processes. Moreover, the use of heated propane as the solvent, as used in processes like the VAPEX process, can cause asphaltene plugging of the well, resulting in a substantial loss of recoverable oil. Further, surfactants typically are cost ineffective due to rock absorption rates.
While the application of heat or chemicals, either directly to the reservoir or via the injection of heated solvents, has a demonstrated effect in mobilizing oil for extraction, each of the above processes suffers from the disadvantage of having either high energy requirements for the generation of the steam and/or heated solvents or a damaging reaction. The energy requirements are typically met through the burning of large amounts of fuel, usually natural gas. This leads to the emission of enormous amounts of greenhouse gases such as carbon dioxide. For example, a 100,000 barrels (bbl) of oil per day SAGD facility requires 200,000-300,000 bbl water per day to be converted into steam. Thus, to recover 100,000 bbl oil per day using a natural gas burner system results in producing more than 12 million pounds per day of carbon dioxide emissions.
Therefore there is a need for an energy efficient and cost effective process for stimulating production of oil, and which does not suffer from the aforementioned problems, such as asphaltene deposition/plugging.